This invention relates to the aerobic biodegradation of alkyl ethers by employing biological organisms in which microbial growth of the organisms has been enhanced by fatty acids. More particularly, this invention relates to the biodegradation of ether-based compounds, such as methyl-tertiary-butyl ether (MTBE), in aqueous systems by treatment of the aqueous systems with microorganisms the growth of which has been stimulated by the presence of a fatty acid such as lactate.
Methyl-tertiary-butyl ether (MTBE) is a fuel oxygenate that is added to gasoline to enhance octane levels, increase burning efficiency and reduce the emission of atmospheric pollutants. Oxygenated compounds such as this have become common in recent years as air quality standards have become more stringent. MTBE is especially popular due to its low production cost, high octane rating, evaporative qualities and solubility with other gasoline components. In general, ether-based compounds such as MTBE, especially those compounds which have only one ether linkage and without other functional groups, are chemically stable and thus not easily degradable. These compounds are also soluble in water and absorb poorly to soil. One characteristic of MTBE, for example, is that it rapidly migrates to ground water through soil. Thus, due to its increased usage and chemical characteristics, MTBE is showing up in greater quantities in drinking water, groundwater, surface waters, and waste water. Although the effects of MTBE on humans are still under study, in the very least, when present in large enough quantities, MTBE in drinking water can result in unpleasant taste and odor. Therefore, as the use of MTBE continues to increase, there is a heightened need to develop processes that can effectively, but cost efficiently, reduce levels of MTBE on a large enough scale to be used in water purification.
The four most common methods for removing MTBE from water are air stripping, absorption, advanced oxidation and hydrophobic membrane separation. Each has inherent drawbacks. For example, since MTBE is a highly water soluble polar compound, high air to water ratios are required in the air stripping process, and, consequently, the use of air stripping may not be economical, especially on large scales as is necessary for water purification. Likewise, membrane separation is considered too expensive for use in general water treatment applications. Notwithstanding these economic drawbacks, there are environmental drawbacks as well. Although air stripping and absorption to particles such as granular activated carbon (GAC) result in remediation of the target contaminant, these methods suffer from the fact that they simply transfer the contaminant to another medium, either to the atmosphere (air stripping) or to solid medium (absorption) which must then be disposed of as hazardous waste. In other instances, such as in some advanced oxidation processes that utilize ozone and ozone/peroxide, the processes result in undesirable by-products such as bromate.
The drawbacks associated with the most common methods of hydrocarbon removal from water have led to the investigation of other purification processes. Biological degradation (biodegradation) is widely used for the treatment of many pollutants. If the target compound is biodegradable, biological treatment is recognized as being the most cost effective treatment method. For this reason many recent processes for removing MTBE and similar ether-based compounds from water have focused on biodegradation techniques. As mentioned above, alkyl ethers such as MTBE are not easily biodegraded. However, it is known that other compounds exhibiting similar biodegradation characteristics to alkyl ethers, such as trichloroethylene (TCE) which is a chlorinated solvent, can be degraded through a process known as xe2x80x9ccometabolic degradationxe2x80x9d or biodegradation. Cometabolic degradation, also referred to as xe2x80x9cco-metabolismxe2x80x9d, is a process in which enzymes that serve a primary metabolic function in a microorganism can be used to break down a target compound that otherwise would not be biodegradable. In such cometabolic processes, the target compound undergoes metabolic transformation while a second compound serves as a primary energy or carbon source.
Often, the success or effectiveness of biodegradation is dependent upon key factors being simultaneously present. First, microorganisms must be present, either naturally or by addition. Second, there must be oxygen, water and trace minerals present available to permit the microorganisms to be metabolically active. Third, there must be available sufficient quantities of biologically utilizable nutrients or xe2x80x9cfoodxe2x80x9d to enable the microbial population to grow and survive. In the industry, these nutrients are known as the xe2x80x9cgrowth substratexe2x80x9d or xe2x80x9cenergy sourcexe2x80x9d. In many cases, the growth substrate or energy source is the target compound itself. In other cases, the target compound will not support bacterial growth by itself and an additional growth substrate must be added to allow the degrading microorganism to grow and cometabolize the target compound. Alkyl ethers, in particular MTBE, have been utilized in both instances. Specifically, in some instances MTBE is merely the target of a biodegradation process, while in other instances, not only is MTBE the target of a biodegradation process, but MTBE is also the growth substrate utilized in the biodegradation process.
U.S. Pat. No. 5,750,364, illustrates use of alkyl ethers directly as growth substrates. This patent teaches a bacterial culture prepared from an ether containing a t-butyl moiety, such as MTBE, and an activated sludge. Specifically, a solution is formed by combining a mineral nutrient solution with biosludge recovered from a waste water treatment plant. The resulting solution is then subjected to an enrichment process that typically lasts from about one month to about one year during which the resulting solution is flushed with O2; combined with a branched alkyl ether; stirred mechanically; skimmed to remove a portion of the supernatant; and replenished with additional mineral solution and ether. The patent teaches that following this enrichment process, the culture becomes active in consistently degrading MTBE concentrations in the supernatant. One drawback to the prior art method of growing microorganism directly on alkyl ethers is that the growth of the microorganism is sporadic and processes dependent on the growth of microorganisms directly on the alkyl ether are inherently unreliable. Often, the result is identification of microbes that can be grown and are effective on a laboratory scale in small quantities, but are not easily reproduced in large enough quantities to be viable for large scale water purification operations, such as is used by municipalities. Further, although the microorganisms might be effective in degrading the target compound, the microorganisms do not grow prolifically in the presence of the target compound. Thus, growth and maintenance of such microorganisms, when possible, is typically time consuming and expensive. For example, in U.S. Pat. No. 5,750,364, referenced above, the process for growing an initial colony of microorganisms requires a minimum of four months.
The prior art has focused on the cometabolism of poorly degraded compound such as chlorinated solvents and alkyl ethers. For example, U.S. Pat. No. 4,713,343 discloses a method and apparatus for treating ground water contaminated with halogenated aliphatic hydrocarbon compounds. In the process, bacteria that can aerobically degrade low molecular weight chlorinated solvents utilizing monooxygenase are combined with the chlorinated solvents in the presence of a low molecular weight alkaynes and oxygen.
In another example, U.S. Pat. 5,814,514 teaches the use of propane-oxidizing bacteria that produce propane monooxygenase (PMO). Specifically, the patent teaches that since PMO is the enzyme primarily responsible for degradation of ether-based compounds, those bacteria that produce PMO are the most effective in degrading ether-based compounds. Although the patent also teaches various carbon sources that are effective in growing the microorganisms, such as propane, isopropanol, acetone, ethanol, butane, laruia broth, trypticase soy agar, and nutrient broths, including casamino acids and/or yeast extract, the primary focus of the teachings is on identification of PMO producing bacteria.
U.S. Pat. No. 4,925,802, teaches the use of innocuous, nontoxic, nongaseous growth substrate that stimulates microorganisms to degrade halogenated aliphatic hydrocarbons by including oxygenase-controlled pathways. Examples of such growth substrate include aromatic amino acids, such as tiyptophan and C10-C20 linear alkanes. The target hydrocarbons are volatile chlorinated aliphatic (VCA) hydrocarbons such as chlorinated ethylene or trichloroethylene. Drawbacks to this patent include the high cost of the growth substrate and the poor water solubility of the co-metabolic compounds.
In those limited instances where various growth substrates have been exploited in biodegradation processes, the processes have yielded unsatisfactory results. In U.S. Pat. No. 4,664,805, the invention is a process that combines an indigenous bacteria with a non-indigenous microorganism and a non-toxic analog or growth substrate. The non-toxic growth substrate is provided to be metabolized by both types of microorganisms and to support growth of both microorganisms. The patent teaches that the specific nature of the growth substrate is a halogenated organic compound containing few or no halogen atoms, such as biphenyl. In the process, an indigenous microorganism capable of decontaminating environments containing halogenated organic compounds is combined with a non-indigenous microorganism that produces metabolic products that support growth of the indigenous microorganism. The growth substrate is present to stimulate growth of both microorganisms. Notwithstanding the complicated nature of the process, one drawback to the process is the need for multiple microorganisms. Additionally, there is some question regarding the toxicity of the growth substrate taught therein.
In none of the prior art of which Applicants are aware are there successful procedures for degradation of certain alkyl ethers such as MTBE in which a substrate is provided to stimulate enzyme production and maximize microbial growth without interfering with the degradation process, especially in the presence of a substrate that is non-toxic. Heretofore, the prior art has not taught a biodegradation process in which the microorganism utilized therein do not rely on the alkyl ether to grow and in which the co-metabolic substrates are very water soluble, inexpensive and ingestible by humans.
It is accordingly one object of the invention to provide a method for the degradation of certain alkyl ethers under aerobic conditions so as to degrade and effectively remove these alkyl ethers from water.
A further object of the invention is to provide a method for the purification of water including purification of ground water and drinking water.
A further object of the invention is to provide a method for the aerobic degradation of MTBE which are present in aqueous systems by the use of microorganisms which have the capacity to metabolize the MTBE.
A further object of this invention is to provide a method for utilizing lactate in conjunction with metals such as iron and manganese to stimulate MTBE biodegradation by iron and manganese oxidizing microorganisms.
A further object of the invention is to provide a method for the purification of water in situ.
A further object of the invention is to provide a growth substrate for the above processes that is non-toxic and water soluble.
A further object of the invention is to provide a growth substrate that can be used to manufacture MTBE degrading microorganisms and to isolate MTBE degrading enzymes produced therefrom.
The subject invention provides a process for the biodegradation of ethers using fatty acid enhanced microbes. When combined with certain microorganisms, such fatty acids permit the control of the MTBE biodegradation process to a fuller extent and a faster rate than achieved in the prior art. In one embodiment, microorganism growth is stimulated and maximized by utilizing lactate as the primary carbon source for the microbes. The fatty acid functions as an inexpensive growth substrate that can be introduced into an aqueous system to quickly maximize the size of the active microbe colony present therein. In a preferred embodiment, the lactate used in the inventive process is food grade. Such a characteristic is desirable since food grade compounds are non-toxic, may be safely consumed by humans, and typically are inexpensive and readily available.
The aqueous systems with which the invention is concerned include drinking water as well as ground water, industrial waste waters and waters such as those produced by an interdiction well designed to intercept and remove a plume of contaminated ground water. The invention is particularly applicable to the treatment of ground water either in situ or by removal of a portion of the water, subjecting it to treatment and then returning the water to its environment. Broadly, however, the invention is concerned with the treatment of water, regardless of location or source, to remove the indicated contaminant using microorganisms stimulated by a substrate that is water soluble, nontoxic and innocuous to the aqueous system.